Enforce non-parallel plan when calling current_schema() in newly-added test

current_schema() gets called in the recently-added regression test from
c5660e0a, and can be used in a parallel context, causing its call to fail
when creating a temporary schema.

Per buildfarm members crake and lapwing.

Discussion: https://postgr.es/m/20190118005949.GD1883@paquier.xyz